The present invention relates to radiometric imaging systems, and, more particularly, to a radiometer which determines the radiation distribution of a scene by scanning the scene with sensitivity lobes of an interferometer.
Radiometric imagers have many real and potential applications. Airborne and spaceborne microwave imagers can be used for: mapping terrestial, planetary and oceanographic features; measuring atmospheric water vapor, rain, and sea surface temperature; and assessing hydrographic phenomena, and meteorologic and surface conditions below clouds or rain.
By way of example, present geosynchronous satellite radiometric meteorological mappers are limited to visible and infrared frequencies which at best weakly penetrate cloud cover. Thus, present systems are precluded from providing continuous observation of the Earth's surface. Portions of the microwave spectrum (1 mm to 1 m wavelengths, herein) readily penetrates cloud cover. Therefore, a microwave radiometer could provide all-weather continuous imaging of the Earth's surface for meteorological and other applications.
Microwave radiometric imagers which use large parabolic dish reflectors are well known. The mass and size of the dishes are related to the angular resolution required and the wavelength to be received. To achieve the resolution required for the applications and wavelengths previously discussed requires very large and heavy structures. Accordingly, these reflectors are generally impractical for remote radio frequency applications.
Analogous problems of bulk and weight have been addressed in radar systems and the solutions have included synthetic aperture radar (SAR). However, the SAR systems are based on reflections of coherent radiation they produce. Radiometric systems are dependent on target emissions, and in general, these are broad-band and incoherent.
What is needed is a practical radiometric imaging system which provides images of high resolution and fidelity relative to bulk and power requirements. It is further important that such a system be adaptable for satellite imaging from geosynchronous orbit.